High voltage ac machine winding with grounded neutral circuit

ABSTRACT

An electric high voltage AC machine intended to be directly connected to a distribution or transmission network ( 16 ) comprises at least one winding. This winding comprises at least one current-carrying conductor, a first layer having semiconducting properties provided around said conductor, a solid insulating layer provided around said first layer, and a second layer having semiconducting properties provided around said insulating layer. In addition grounding means ( 18, 24, 26, 28 ) are provided to connect at least one point of said winding to ground.

[0001] The present invention relates to an electric high voltage ACmachine intended to be directly connected to a distribution ortransmission network, said machine comprising at least one winding.

[0002] Such generators with a rated voltage of up to 36 kV is describedby Paul R. Siedler, “36 kV Generators Arise from Insulation Research”,Electrical World, Oct. 15, 1932, pp. 524-527. These generators comprisewindings formed of medium voltage insulated conductors whereininsulation is subdivided into various layers of different dielectricconstants. The insulating material used is formed of variouscombinations of the three components of micafolium-mica, varnish andpaper.

[0003] In a publication by Power Research Institute, EPRI, EL-3391,April 1984 a generator concept is proposed for providing such highvoltages that the generator can be directly connected to a power networkwithout any intermediate transformer. Such a generator was supposed tocomprise a superconducting rotor. The magnetization capacity of thesuperconducting field would then make it possible to use air gapwindings of sufficient thickness for withstanding the electric forces.The proposed rotor is, however, of a complicated structure with a verythick insulation which considerably increases the size of the machine.In addition thereto special measures have to be taken for insulating andcooling the coil end sections.

[0004] By electric high voltage AC machines is meant, according to thepresent invention, rotating electric machines like generators in powerstations for production of electric power, double-fed machines, outerpole machines, synchronous machines, asynchronous converter cascades, aswell as power transformers. For connecting such machines, except fortransformers, to distribution and transmission networks, in thefollowing commonly referred to as power networks, a transformer has sofar been needed for transforming the voltage up to the network level,that is in the range of 130-400 kV.

[0005] By manufacturing the winding of these machines of an insulatedelectric high voltage conductor with a solid insulation of similarstructure as cables used for power transmission the voltage of themachine can be increased to such levels that the machines can bedirectly connected to any power network without an intermediatetransformer. Thus this transformer can be omitted. Typical working rangefor these machines is 30-800 kV.

[0006] For this kind of machines special attention has to be paid togrounding problems.

[0007] Grounding of generator systems and other similar electricalsystems implies intentional measures for connecting an electric systemto ground potential. When the so-called neutral point of the system isavailable it is often connected to ground, directly or through asuitable impedance. It also happens that other points in the system areconnected to ground. If one point in the system is grounded the completesystem is grounded as long as the galvanic connection extends.

[0008] The grounding principle used is determined by the design of thesystem. For a system including a generator directly connected to a Y-Δconnected step-up-transformer with the Δ-winding at the generatorvoltage the following grounding alternatives are most common.

[0009] High resistance grounding

[0010] No grounding

[0011] Resonant grounding.

[0012] High resistance grounding is normally realized by connection of alow ohmic resistor in the secondary of a distribution transformer withthe primary winding of the transformer connected from the generatorneutral point to ground. Such prior art grounding is illustrated in FIG.1, which shows a generator 2 connected by a Y-Δ connected step-uptransformer 3 to a network 9. The primary 11 of a distributiontransformer is connected between the neutral point of the generator 2and ground. In the secondary 10 of the transformer a resistor 12 isconnected.

[0013] The same kind of grounding can, of course, be obtained byinstalling a high ohmic resistor directly between the generator neutralpoint and ground.

[0014] An ungrounded electric system lacks any intentional connection toground. Thus an ungrounded generator has no connection between itsneutral point and ground, except for possible voltage transformers forfeeding different relays and instruments.

[0015] Resonant grounding is normally also realized as illustrated inFIG. 1 with the resistor 12 replaced by a reactor 12 a. The reactorreactance is chosen such that the capacitive current during a line toground fault is neutralized by an equal component of inductive currentcontributed for by the reactor 12 a.

[0016] Also low resistance or low impedance grounding and effectivegrounding of the above systems are possible. Low resistance or lowimpedance grounding will result in lower transient overvoltages buthigher ground fault currents, which can cause internal damages to themachine.

[0017] Low resistance grounding is achieved by the intentional insertionof a resistance between the generator neutral and ground. The resistancemay be inserted either directly in connection to ground or indirectly,in the secondary of a transformer whose primary is connected betweengenerator neutral and ground, cf. FIG. 1.

[0018] Low impedance grounding, that is low inductance grounding isaccomplished in the same way as low resistance grounding with thesubstitution of an inductor for the resistor. The value of the inductorin ohms is less than that required for resonant grounding, as discussedabove.

[0019] For systems comprising several generators connected to a commonfeeding line or bus with circuit breakers between the generatorterminals and the common bus low resistance or low impedance groundingis suitable.

[0020] Effectively grounding the neutral of a generator hassubstantially the same advantages and disadvantages as the lowresistance or low impedance grounding with some differences.

[0021] A system is said to be effectively grounded if certain impedancerequirements, which restricts the size of the grounding impedance, arefulfilled. In an effectively grounded system the maximum phase-to-groundvoltage in unfaulted phases, in case of a ground fault, are limited to80% of phase-to-phase voltage.

[0022] A power system network is mainly grounded through groundconnections of neutral points of transformers in the system and caninclude no impedance (except for contact resistances), so-called directgrounding, or have a certain impedance.

[0023] Previously known grounding techniques are described in e.g. thepublication IEEE C62.92-1989, IEEE Guide for the Application of NeutralGrounding in Electrical Utility Systems, Part II—Grounding ofSynchronous Systems, published by the Institute of Electrical andElectronics Engineers, New York, USA, September, 1989.

[0024] If the generator neutral is grounded through a low resistance orinductance as discussed above, a path is formed for third harmoniccurrents from the generator neutral to ground. If a directly grounded orlow-impedance grounded transformer winding or another low-impedancegrounded generator is directly connected to the generator, the thirdharmonic currents will circulate therebetween under normal conditions.

[0025] Techniques for solving the problems of third harmonic currents ingenerator- and motor-operation of AC electric machines of the kind towhich the present invention relates are described in Sweedish patentapplication Ser. Nos. 9602078-9 and 97003-9.

[0026] The purpose of the present invention is to provide an electrichigh voltage AC machine suitable for direct connection to distributionor transmission networks as indicated above, which machine is providedwith grounding means suitable for different uses and operatingconditions of the machine.

[0027] This purpose is obtained with an electric high voltage AC machineof the kind defined in the introductory portion of the description andhaving the characterising features of claim 1.

[0028] An important advantage of the machine according to the inventionresides in the fact that the electric field is nearly equal to zero inthe end region of the windings outside the second layer withsemiconducting properties. Thus no electric fields need to be controlledoutside the winding and no field concentrations can be formed, neitherwithin the sheet, nor in winding end regions, nor in transitionstherebetween.

[0029] According to an advantageous embodiment of the machine accordingto the invention at least two adjacent layers have substantially equalthermal expansion coefficients. In this way defects, cracks or the likeas a result of thermal motions in the winding, are avoided.

[0030] According to another advantageous embodiment of the machineaccording to the invention said grounding means comprise means for lowresistance grounding of the winding. In this way transient overvoltagesas well as the ground fault current can be limited to moderate values.

[0031] According to still another advantageous embodiment of the machineaccording to the invention, wherein the machine has a Y-connectedwinding, the neutral point of which being available, high resistancegrounding means comprise a resistor connected in the secondary of atransformer whose primary is connected between the neutral point andground. In this way the resistor used in the secondary of thetransformer is of comparatively low ohmic value and of ruggedconstruction. Sufficient damping to reduce transient overvoltages tosafe levels can be achieved with a properly sized resistor. Further,mechanical stresses and fault damages are limited during line-to-groundfaults by the restriction of the fault current. Such a grounding deviceis also more economical than direct insertion of a high ohmic resistorbetween the generator neutral and ground.

[0032] According to another advantageous embodiment of the machineaccording to the invention, wherein the machine has a Y-connectedwinding the neutral point of which being available, the grounding meanscomprises a reactor connected in the secondary of a transformer whoseprimary is connected between the neutral point and ground, said reactorhaving characteristics such that the capacitive current during a groundfault is substantially neutralized by an equal component of inductivecurrent contributed for by the reactor. In this way the net faultcurrent is reduced to a low value by the parallel resonant circuit thusformed, and the current is essentially in phase with the fault voltage.The voltage recovery on the faulted phase is very slow in this case andaccordingly any ground fault of a transient nature will automatically beextinguished in a resonant grounded system.

[0033] According to still other advantageous embodiments of the machineaccording to the invention the grounding means comprise a Y-Δ groundingtransformer or a so-called zigzag grounding transformer connected to thenetwork side of the machine. The use of such grounding transformers areequivalent to low inductance or low resistance grounding with respect tofault current levels and transient overvoltages.

[0034] To explain the invention in more detail embodiments of themachine according to the invention, chosen as examples, will now bedescribed more in detail with reference to FIG. 2-11 on the accompanyingdrawings on which

[0035]FIG. 1 illustrates prior art grounding of a synchronous generator,

[0036]FIG. 2 shows an example of the insulated conductor used in thewindings of the machine according to the invention,

[0037]FIG. 3 shows an ungrounded three-phase machine in the form of aY-connected generator or motor connected to a power system,

[0038]FIG. 4-13 show different examples of grounding the Y-connectedmachine in FIG. 3,

[0039]FIG. 14 shows a machine according to the invention in the form ofa Δ-connected generator or motor connected to a power system, and

[0040]FIG. 15 illustrates the use of a grounding transformer in thesystem shown in FIG. 14.

[0041] In FIG. 2 an example is shown of an insulated conductor, whichcan be used in the windings of the machine according to the invention.Such an insulated conductor comprises at least one conductor 4 composedof a number of non-insulated and possibly insulated strands 5. Aroundthe conductor 4 there is an inner semiconducting layer 6, which is incontact with at least some of the non-insulated strands 5. Thissemiconducting layer 6 is in its turn surrounded by the main insulationof the cable in the form of an extruded solid insulating layer 7. Theinsulating layer is surrounded by an external semiconducting layer 8.The conductor area of the cable can vary between 80 and 3000 mm² and theexternal diameter of the cable between 20 and 250 mm.

[0042]FIG. 3 shows schematically an ungrounded electric high voltage ACmachine in the form of a Y-connected generator or motor 14 directlyconnected to a power system 16.

[0043]FIG. 4 shows grounding means in the form of an overvoltageprotector, like a non-linear resistance arrester 18, connected betweenthe neutral point 20 of the Y-connected machine 14 and ground. Such anon-linear resistance arrester 18 connected to the neutral pointprotects the insulated conductor used in the machine windings againsttransient overvoltages, such as overvoltages caused by a stroke oflightning.

[0044]FIG. 5 shows an embodiment with a high ohmic resistor 22 connectedin parallel to the non-linear resistance arrester 18. The non-linearresistance arrester 18 is functioning in the same way in this embodimentas in the embodiment shown in FIG. 4 and with the resistor 22 asensitive detection of ground faults by measuring the voltage across theresistor 22 is realised.

[0045]FIG. 6 shows an embodiment with high resistance grounding of theneutral point 20. In this embodiment a technique similar to the priorart described in connection with FIG. 1 is used. Thus a resistor 24 isconnected to the secondary 26 of a transformer with the primary winding28 of the transformer connected from the neutral point 20 of the machine14 to ground. The resistor 24 is comparatively low ohmic and of ruggedconstruction, as compared to a high ohmic resistor which would be neededfor direct connection between the neutral point 20 and ground forobtaining the same result. The voltage class of the resistor canconsequently be reduced. Also in this case a non-linear resistancearrester 18 is connected in parallel to the primary winding 28. Withthis embodiment mechanical stresses and fault damages are limited duringline-to-ground faults by restricting the fault current. Transientovervoltages are limited to safe levels and the grounding device is moreeconomical than direct insertion of a resistor.

[0046] Resonant grounding of the machine can be realised in a similarway by replacing the resistor 24 by a reactor having characteristicssuch that the capacitive current during a line-to-ground fault isneutralized by an equal component of inductive current contributed forby the reactor. Thus the net fault current is reduced by the parallelresonant circuit thus formed and the current will be essentially inphase with the fault voltage. After extinction of the fault the voltagerecovery on the faulted phase will be very slow and determined by theratio of inductive reactance to the effective resistance of thetransformer/reactor combination. Accordingly any ground fault oftransient nature will automatically be extinguished in such a resonantgrounded system. Thus such resonant grounding means limits the groundfault current to practically zero, thus minimising the mechanicalstresses Further continued operation of the machine can be permittedafter the occurrence of a phase-to-ground fault until an orderlyshutdown can be arranged.

[0047]FIG. 7 shows an embodiment with a non-linear resistance arrester18 connected between the neutral point 20 and ground and a groundingtransformer 30 connected on the network side of the machine 14. Thegrounding transformer 30 is of Y-Δ design with the neutral point of theY-connection connected to ground, whereas the Δ-winding is isolated.Grounding transformers are normally used in systems which are ungroundedor which have a high impedance ground connection. As a system componentthe grounding transformer carries no load and does not affect the normalsystem behaviour. When unbalances occur the grounding transformerprovides a low impedance in the zero sequence network. The groundingtransformer is in this way equivalent to a low inductance or lowresistance grounding with respect to fault current levels and transientovervoltages.

[0048] The grounding transformer can also be a so-called zigzagtransformer with special winding arrangements, see e.g. Paul M.Anderson, “Analysis of Faulted Power Systems”, The Iowa State UniversityPress/Ames, 1983, pp. 255-257.

[0049] Also a possible auxiliary power transformer can be used for suchgrounding purposes.

[0050]FIG. 8 shows an embodiment with a low ohmic resistor 32 connectedbetween the neutral point 20 of the machine 14 and ground. The mainadvantage of such a low resistance grounding is the ability to limittransient and temporary overvoltages. The currents will, however, behigher in case of single phase ground faults. Also third harmoniccurrents will be higher in undisturbed operation.

[0051]FIG. 9 shows an alternative embodiment of the machine according tothe invention in which the resistor 32 is replaced by a low inductanceinductor 34 connected between the neutral point 20 and ground. Lowinductance grounding works essentially in the same way as low ohmicgrounding. The value of the inductor 34 in ohms is less than thatrequired for resonant grounding, cf. description of FIG. 6.

[0052] As an alternative to the direct connection between the neutralpoint 20 and ground of the resistor 32 or the inductor 34, they may beindirectly connected with the aid of a transformer whose primary isconnected between the neutral point 20 and ground and whose secondarycontains the resistor or inductor, cf. the description of FIG. 6.

[0053] In FIG. 10 an embodiment is shown comprising two impedances 36and 38 connected in series between the neutral point 20 of the machine14 and ground, the impedance 36 having a low impedance value and theimpedance 38 a high impedance value. The impedance 38 can beshort-circuited by a short-circuiting device 40. In normal operation theshort-circuiting device 40 is open in order to minimize third harmoniccurrents. In case of a ground fault the short-circuiting device 40 iscontrolled to short-circuit the impedance 38 and the potential in theneutral point 20 will be low and the current to ground comparativelyhigh.

[0054] In case of an internal ground fault in the machine 14 theimpedance 38 is not short-circuited. As a consequence the voltage willbe high in the neutral point 20 but the current to ground will belimited. In such a situation this is to prefer since a high current cangive rise to damages in this case.

[0055] To be able to cope with the problems arising from third harmonicswhen directly connecting an AC electric machine to a three-phase powernetwork, i.e. when no step-up transformer is used between the machineand the network, grounding means in the form of a suppression filter 35,37, tuned to the third harmonic together with an overvoltage protector39 can be used, see FIG. 11. The filter thus comprises a parallelresonance circuit consisting of an inductor 35 and a capacitivereactance 37. The dimensioning of the filter 35, 37 and its overvoltageprotector 39 is such that the parallel circuit is capable of absorbingthird harmonics from the machine 14 during normal operation, yetlimiting transient and temporary overvoltages. In case of a fault theovervoltage protector 39 will limit the fault voltage such that thefault current flows through the overvoltage protector 39 if the fault isconsiderable. In case of a single-phase ground fault the currents willbe higher as compared to e.g. the case of high resistance groundingsince the fundamental impedance is low.

[0056] In FIG. 12 an embodiment is shown wherein the grounding meanscomprises a detuned switchable third harmonics depression filterconnected in parallel to an overvoltage protector 40. Such filters canbe realised in several different ways. FIG. 12 shows an examplecomprising two inductors 42, 44 connected in series and a capacitor 46connected in parallel to the series-connected inductors 42, 44. Furthera short-circuiting device 48 is connected across the inductor 44.

[0057] The short-circuiting device 48 is controllable to change thecharacteristic of the filter by short-circuiting the inductor 44 when arisk for third harmonic resonance between the filter and the machine 14and network 16 is detected. This is described more in detail in Swedishpatent application 9700347-9. In this way third harmonic currents arestrongly limited in normal operation. Transient and temporaryovervoltages will be limited and the currents will be higher in case ofa single-phase ground fault in the same way as described in connectionwith FIG. 11.

[0058]FIG. 13 shows an embodiment wherein the neutral point 20 of themachine 14 is directly connected to ground, at 21. Such direct groundinglimits transient and temporary overvoltages but results in high currentsin case of ground faults. Third harmonic current flow from the neutral20 of the machine to ground will be comparatively high in normaloperation.

[0059] As a further alternative the grounding means of the machineaccording to the invention can comprise an active circuit for providinga connection of the neutral point to ground having desirable impedanceproperties.

[0060] In FIG. 14 a Δ-connected three-phase machine 50 is shown directlyconnected to the distribution or transmission network 16.

[0061] In such a situation a grounding transformer of the same kind asthe one used in the embodiment shown in FIG. 7 can be connected on thenetwork side of the machine 50.

[0062] As in the embodiment of FIG. 7 the grounding transformer can be aY-Δ-connected transformer with the neutral point of the Y-connectionground, or a so called zigzag grounding transformer, that is aZ-0-connected transformer with the Z grounded. The grounding transformerwill limit temporary overvoltages.

[0063] As in the embodiment of FIG. 7 a possible auxiliary powertransformer can also be used for this purpose.

1. An electric high voltage AC machine, intended to be directlyconnected to a distribution or transmission network (16), said machineincluding at least one winding comprising at least one insulatedcurrent-carrying conductor (4), characterized in that a first layer (6)having semi-conducting properties is provided around said conductor (4),a solid insulating layer (7) is provided around said first layer, and asecond layer (8) having semi-conducting properties is provided aroundsaid insulating layer, and in that grounding means(18,21,22,24,26.28,30,32,34,35,36,37,38,39,40,42,44,46,48, 52) areprovided to connect at least one point of said winding to ground.
 2. Themachine according to claim 1, characterized in that the potential ofsaid first layer is substantially equal to the potential of theconductor.
 3. The machine according to claim 1 or 2, characterized inthat said second layer is arranged to constitute substantially anequipotential surface surrounding said conductor.
 4. The machineaccording to claim 3, characterized in that said second layer isconnected to a predetermined potential.
 5. The machine according toclaim 4, characterized in that said predetermined potential is groundpotential.
 6. The machine according to any one of the claims 1, 2, 3, 4or 5, characterized in that at least two adjacent layers havesubstantially equal thermal expansion coefficients.
 7. The machineaccording to any one of the preceding claims, characterized in that saidcurrent-carrying conductor comprises a number of strands, only aminority of said strands being non-isolated from each other.
 8. Themachine according to any one of the preceding claims, characterized inthat each of said three layers is fixed connected to adjacent layeralong substantially the whole connecting surface.
 9. An electric ACmachine having a magnetic circuit for high voltage comprising a magneticcore and at least one winding, characterized in that said winding isformed of a cable comprising one or more current-carrying conductors,each conductor having a number of strands, an inner semi-conductinglayer provided around each conductor, an insulating layer of solidinsulating material provided around said inner semi-conducting layer,and an outer semi-conducting layer provided around said insulatinglayer, and in that grounding means are provided to connect at least onepoint of said winding to ground.
 10. The machine according to claim 9,characterized in that said cable also comprises a metall shield and asheath.
 11. The machine according to any one of the preceding claims,characterized in that said grounding means comprise means for directgrounding of the winding.
 12. The machine according to any one of theclaims 1 through 10, characterized in that said grounding means comprisemeans for low-resistance grounding of the winding.
 13. The machineaccording to claim 12, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidlow-resistance grounding means comprise a low-resistance resistorconnected between the neutral point and ground.
 14. The machineaccording to claim 12, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidlow-resistance grounding means comprise a resistor connected in thesecondary of a transformer whose primary is connected between theneutral point and ground.
 15. The machine according to any one of theclaims 1 through 10, characterized in that said grounding means comprisemeans for low-inductance grounding of the winding.
 16. The machineaccording to claim 15, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidlow-inductance grounding means comprise a low-inductance inductorconnected between the neutral point and ground.
 17. The machineaccording to claim 15, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidlow-inductance grounding means comprise an inductor connected in thesecondary of a transformer whose primary is connected between theneutral point and ground.
 18. The machine according to any one of theclaims 1 through 10, characterized in that said grounding means comprisemeans for high-resistance grounding of the winding.
 19. The machineaccording to claim 18, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidhigh-resistance grounding means comprise a high-resistance resistorconnected between the neutral point and ground.
 20. The machineaccording to claim 18, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidhigh-resistance grounding means comprise a resistor connected in thesecondary of a transformer whose primary is connected between theneutral point and ground.
 21. The machine according to any one of theclaims 1 through 10, characterized in that said grounding means comprisemeans for high-inductance grounding of the winding.
 22. The machineaccording to claim 21, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidhigh-inductance grounding means comprise a high-inductance inductorconnected between the neutral point and ground.
 23. The machineaccording to claim 21, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidhigh-inductance grounding means comprise an inductor connected in thesecondary of a transformer whose primary is connected between theneutral point and ground.
 24. The machine according to any one of theclaims 1 through 10, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidgrounding means comprise a reactor connected in the secondary of atransformer whose primary is connected between the neutral point andground, said reactor having characteristics such that the capacitivecurrent during a ground fault is substantially neutralized by an equalcomponent of inductive current contributed for by the reactor.
 25. Themachine according to any one of the claims 1 through 10, characterizedin that said grounding means comprise means for changing the impedanceof the connection to ground in response to a ground fault.
 26. Themachine according to any one of the claims 1 through 10, characterizedin that said grounding means comprise an active circuit.
 27. The machineaccording to any one of the claims 1 through 10, characterized in thatsaid grounding means comprise a Y-Δ grounding transformer connected tothe network side of the machine.
 28. The machine according to any one ofthe claims 1 through 10, characterized in that said grounding meanscomprise a so-called zigzag grounding transformer connected to thenetwork side of the machine.
 29. The machine according to any one of theclaims 1 through 10, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidgrounding means comprise a suppression filter tuned for the n:thharmonic.
 30. The machine according to any one of the claims 1 through10, said machine having a Y-connected winding the neutral point of whichbeing available, characterized in that said grounding means comprise aswitchable suppression filter detuned for the n:th harmonic.
 31. Themachine according to claim 29 or 30, characterized in that said n:thharmonic is the third harmonic.
 32. The machine according to any one ofthe claims 1 through 10, said machine having a Y-connected winding theneutral point of which being available, characterized in that saidgrounding means comprise an overvoltage protector connected between saidneutral point and ground.
 33. The machine according to any one of theclaims 18 through 31, said machine having a Y-connected winding theneutral point of which being available, characterized in that anovervoltage protector is connected between said neutral point and groundin parallel to said grounding means.
 34. A distribution or transmissionnetwork, characterized in that it comprises at least one machineaccording to any one of the claims 1 through 33.